Numerical investigation of droplet pre-dispersion in a monodisperse droplet spray dryer

Monodisperse droplet spray dryers have great advantages in particle formation through spray drying because of their ability to produce uniform sized particles. Experimental analyses of this system have shown that droplets atomized through the piezoceramic nozzle need to be sufficiently well dispersed before entering the drying chamber to achieve sufficiently dried particles. However, the dispersion dynamics cannot be readily observed because of experimental limitations, and key factors influencing the dispersion state currently remain unclear. This study carried out numerical simulations for droplet dispersions in the dispersion chamber, which allow this important process to be visualized. The systematic and quantitative analyses on the dispersion states provide valuable data for improving the design of the dispersion chamber, and optimizing the spray drying operation.     

Numerical prediction of droplet dynamics in turbulent flow,using the level set method

In the present article, the droplet dynamics in turbulent flow is numerically predicted. The modelling is based on an interfacial marker-level set (IMLS) method, coupled with the Reynolds-averaged Navier–Stokes (RANS) equations to predict the dynamics of turbulent two-phase flow. The governing equations for time-dependent, two-dimensional and incompressible two-phase flow are described in both phases and solved separately using a control volume approach on structured cell-centred collocated grids. The topological changes of the interface are predicted by applying the level set approach. The kinematic and dynamic conditions on the interface separating the two phases are satisfied. The numerical method proposed is validated against a well-known computational fluid dynamics problem. Further, the deformation and breakup of a single droplet either suddenly moved in air or exposed to turbulent stream are numerically investigated. In general, the developed numerical method demonstrates remarkable capability in predicting the characteristics of complex turbulent two-phase flows.     

Influence of Axisymmetric Assumptions on Large Eddy Simulations of a Confined Jet and a Swirl Flow

Axisymmetric geometries can be found in many practical flow applications. In the attempt to predict these flows numerically, RANS flow solvers can decrease the computational efforts dramatically by taking this axisymmetry into account and by computing only a pie-segment of the flow. However, the extension of the concept of axisymmetric flows to LES computations is not straightforward, since the boundary conditions on the axis of symmetry are altering the instantaneous flow field. In this study, the influence of the introduction of an axis of symmetry to LES computations is assessed by computations of a flow with and without swirl over an axisymmetric expansion. The LES computations are performed on a full three-dimensional and a 90° segment of the geometry. The results are compared and the influence of the axis put into relation with the gain in computational costs.     

An experimental investigation of droplet evaporation and coalescence in a simple jet flow

An experimental study of a simple jet flow, which contains a dispersion of fine droplets, has been carried out in order to investigate the effect of turbulence, evaporation and coalescence on the droplet size distributions within the jet. Very little evaporation occurs in the potential core of the jet, while in the far-field, where the potential core has vanished and the droplets disperse more readily, evaporation occurs predominantly in the outer portions of the spray. Evidently, turbulence enhances the evaporation rate of droplets at the edges of the spray, and fresh air entrained from the outer regions increases the evaporative driving force. Coalescence has also been observed within the spray, although this effect is rather subtle compared to the evaporation effect in the dilute jets investigated here. Nevertheless, sufficient measurements have been taken to validate, at least partially, any coalescence models, in addition to any turbulence and evaporation models for dilute poly-disperse sprays.     

大气绕山体的运动及其输移特性的研究

对分层大气山体绕流的流动模式及扩散输移特性进行了数值模拟。采用隐式时间离散方法在贴体网格系统下求解雷诺平均的N-S方程,计算结果描述出大气流动的特征,证实了分层(以Froude数为特征参数,定义为F=U/NH,U为来流风速,N为Brunt-Vaisala频率,H为山体高度)变化对山体绕流流态的影响。数值结果表明:当Froude数大于4.0时,山体绕流的流动不再依赖于大气分层的变化。当Froude数介于4.0和1.0之间时,流场中出现了Lee波,并随着Froude数的进一步减小,流动分离发生及Lee波破碎现象。同时模型也预测了在各种流动模式下大气中夹杂着的污染物绕山体的传输特性,表明大气的分层现象对污染物的分布有着非常重要的影响。     

Numerical and experimental study of an axially induced swirling pipe flow

In-line flow segregators based on axial induction of swirling flow have important applications in chemical, process and petroleum production industries. In the later, the segregation of gas bubbles and/or water droplets dispersed into viscous oil by swirling pipe flow may be beneficial by either providing a pre-separation mechanism (bubble and/or drop coalescer) or, in the case of water-in-oil dispersions, by causing a water-lubricated flow pattern to establish in the pipe (friction reduction). Works addressing these applications are rare in the literature. In this paper, the features and capabilities of swirling pipe flow axially induced by a vane-type swirl generator were investigated both numerically and experimentally. The numerical analysis has been carried out using a commercial CFD package for axial Reynolds numbers less than 2000. Pressure drop, tangential and axial velocity components as well as swirl intensity along a 5 cm i.d. size and 3 m long pipe were computed. Single phase flow experiments have been performed using a water–glycerin solution of 54 mPa s viscosity and 1210 kg/m³ density as working fluid. The numerical predictions of the pressure drop were compared with the experimental data and agreement could be observed within the range of experimental conditions. The experiments confirmed that swirl flow leads to much higher friction factors compared with theoretical values for non-swirl (i.e. purely axial) flow. Furthermore, the addition of a conical trailing edge reduces vortex breakdown. Visualization of the two-phase swirling flow pattern was achieved by adding different amounts of air to the water–glycerin solution upstream the swirl generator.     

液滴在气体介质中剪切破碎的数值模拟研究

液滴变形和破碎是燃料抛撒问题的重要过程.本文将VOF方法和标准k-ε湍流模型组合,建立了计算液滴在气流中变形破碎过程的数值方法.数值模拟了相关的实验,计算得到的液滴破碎过程与实验结果符合较好.在此基础上,分析了几个关键参数(Weber数、Ohnesorge数、液气密度比)对液滴破碎过程的影响.计算结果表明,Weber数...     

Numerical and experimental investigation of a serpentine inlet duct

This article presents a numerical and experimental investigation of the flow inside an ultra-compact, serpentine inlet duct. The numerical analysis used two flow solvers: FLUENT®, a commercial code, and UNS3D, an in-house code. The flow was modelled using the Reynolds-averaged Navier-Stokes equations. The turbulence effects were modelled by using the shear-stress transport k–ω model. The numerical investigation was compared against experimental data obtained in an open-circuit, low-speed wind tunnel in the Fluid Dynamics Laboratory at Texas A&M University. The numerical simulations and experimental testing were performed to reveal the separation points and the strong secondary flow phenomena within the inlet. UNS3D overpredicted the location of the first separation point by 9 mm and the location of the second separation point by 1 mm, while the area-averaged pressure loss coefficient was 5% higher than in the experiment. The numerical results of UNS3D agreed better with the experiment than those of FLUENT.     

An experimental investigation of dispersion in layered porous media

Experiments were run in three linear, homogeneous, nonuniform porous media constructed in lucite columns using spherical glass beads. The columns were also joined end to end to create an in series layered heterogeneous porous media. Each column, all combinations of columns and several permutations were studied with a factorial experimental design to determine the effects of porosity, permeability, velocity, length, and column order upon dispersion. Attempts to predict the heterogeneous results from the homogeneous results were made, and a statistical regression based on the factorial design was calculated. Results showed that no simple averaging procedure accurately predicted the heterogeneous results. The statistical regression showed permeability, velocity, viscosity, length and column order to be significant.     

Effect of the contact-line dynamics on the natural oscillations of a cylindrical droplet

The natural oscillations of a cylindrical droplet of an inviscid liquid surrounded by a different liquid and bounded in the axial direction by solid planes are studied. The motion of the contact line is described using an effective boundary condition. The dependence of the frequency and damping ratio on the capillary parameter is found. It is shown that the fundamental frequency of the translation mode vanishes beginning from a certain value of the capillary parameter. Depending on the ratio of the radial and axial dimensions of the droplet, the fundamental frequency of the axisymmetric mode and modes higher than the translation mode can vanish in a certain range of the capillary parameter. This dependence of the natural oscillation frequencies on the problem parameters allows one to determine the capillary parameter. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 5, pp. 78–86, September–October, 2007.     

Research on the particle dispersion in the particulate two-phase round jet

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Numerical and experimental investigation of turbulent flow in a rectangular duct

Details of the turbulent flow in a 1:8 aspect ratio rectangular duct at a Reynolds number of approximately 5800 were investigated both numerically and experimentally. The three-dimensional mean velocity field and the normal stresses were measured at a position 50 hydraulic diameters downstream from the inlet using laser doppler velocimetry (LDV). Numerical simulations were carried out for the same flow case assuming fully developed conditions by imposing cyclic boundary conditions in the main flow direction. The numerical approach was based on the finite volume technique with a non-staggered grid arrangement and the SIMPLEC algorithm. Results have been obtained with a linear and a non-linear (Speziale) k–ε model, combined with the Lam–Bremhorst damping functions for low Reynolds numbers. The secondary flow patterns, as well as the magnitude of the main flow and overall parameters predicted by the non-linear k–ε model, show good agreement with the experimental results. However, the simulations provide less anisotropy in the normal stresses than the measurements. Also, the magnitudes of the secondary velocities close to the duct corners are underestimated. © 1998 John Wiley & Sons, Ltd.     

带尾翼水下自然超空泡射弹数值模拟研究

基于Navier-Stokes方程,考虑气液间相变,采用大涡模拟湍流模型,PISO算法及VOF方法,利用自行开发的程序对绕三维带尾翼水下自然超空泡射弹的非定常空泡流动进行了数值模拟研究.通过计算结果得到超空泡从射弹头部初生至完全包裹弹身的形成过程.在此基础上,计算不同攻角条件下的超空泡形态,并给出射弹表面空泡厚度分布曲线,分析不同攻角对超空泡形态特性的影响规律,以及在零攻角条件下,分析尾翼对空泡形态的影响.另外计算不同空化数条件下的超空泡无量纲长度和厚度,将计算结果与实验数据进行对比,两者吻合较好.研究结果为进一步研究水下高速射弹的水动力特性和弹道特性问题提供理论基础.     

多循环脉冲爆震发动机流场数值研究

针对二维带有收敛扩张喷管的脉冲爆震发动机模型，采用带基元化学反应的Euler方程组和H2、空气的9组分20基元反应，对发动机在前六个工作循环的流场进行了数值模拟。通过对前几个循环流场进行比较，发现脉冲爆震发动机在第五个循环后流场就基本稳定，单循环得到的流场和多循环稳定后的流场有很大的差别，同时喷管对发动机内流场影响特别大。     

Application of a droplet cluster to visualize microscale gas and liquid flows

The possibility of using microdroplets, which form a dissipative droplet-cluster structure, as tracers localized in the boundary layer of a gas adjoining a liquid-gas interface is experimentally demonstrated. Examples of the visualization of boundary layer flow fields generated by both processes in the gaseous phase and liquid convection are considered.     

Numerical and experimental investigation of flow over a semicircular weir

The water flow over a semicircular weir is investigated numerically and experimentally in this paper. The numerical model solves the Reynolds equation for a mean flow field with thek-ε-turbulent model. To trace the motion of the free surface, the COF method with geometric reconstruction is employed. The velocity of the flow is measured by means of LDV technique. Four types of flow patterns, the position of the separation and reattachment point, the distribution of shear stress on the bed at downstream of the weir are presented and discussed. The numerical results agree well with the experiment data.     

Numerical study of rheological behaviors of a compound droplet in a conical nozzle

The dynamics of compound droplets is more and more attractive because of their applications in a wide range of industrial and natural processes. This study aims to improve the understanding of dynamical rheological behaviors of a compound droplet moving in a nozzle with a conical shape in the downstream region via front-tracking-based simulations. The numerical results show that the compound droplet experiences three stages of deformation: the entrance stage (in front of the conical region), the transit stage (within the conical region), and the exit stage (in the exit of the nozzle). The droplet receives the maximum deformation in the axial direction during the transit stage, and the radially maximum deformation occurs during the exit stage. Because of the acceleration induced by the conical region, the inner droplet of the compound droplet can break up into smaller droplets during the exit stage. To reveal the transition between the finite deformation and the breakup, many parameters including the Capillary number Ca (varied in the range of 0.0125–1.6), the droplet size relative to the nozzle size R₁/R₀ (varied in the range of 0.2–0.9), the droplet radius ratio R₂₁ (varied in the range of 0.3–0.8), the viscosity ratios μ₂₁ and μ₃₁ (varied in the range of 0.05–3.2), the interfacial tension ratio σ₂₁ (varied in the range of 0.125–8.0), the conical angle α (varied in the range of 4°–34°) and the initial location of the inner droplet (i.e. the droplet eccentricity) are considered. From the finite deformation mode, the transition to the breakup mode of the inner droplet occurs when increasing any of Ca, R₁/R₀, R₂₁ and α, or decreasing any of μ₂₁ and σ₂₁. The breakup mode is also enhanced when the inner droplet is initially located closer to the leading side of the outer droplet. However, varying μ₃₁ induces no transition between these modes. The regime diagrams of these modes, based on these parameters, are also proposed.     

Numerical investigation of the spray-mesh-turbulence interactions for high-pressure, evaporating sprays at engine conditions

This work presents a numerical methodology to simulate evaporating, high pressure Diesel sprays using the Eulerian-Lagrangian approach. Specific sub-models were developed to describe the liquid spray injection and breakup, and the influence of the liquid jet on the turbulence viscosity in the vicinity of the nozzle. To reduce the computational time and easily solve the problem of the grid dependency, the possibility to dynamically refine the grid where the fuel-air mixing process takes place was also included.The validity of the proposed approach was firstly verified simulating an evaporating spray in a constant-volume vessel at non-reacting conditions. The availability of a large quantity of experimental data allowed us to investigate in detail the effects of grid size, ambient diffusivity and used spray sub-models. In this way, different guidelines were derived for a successful simulation of the fuel-air mixture formation process. Finally, fuel injection and evaporation were simulated in an optical engine geometry and computed mixture fraction distributions were compared with experimental data.